Modern cellular communication networks typically support numerous user devices, all of which are competing for limited communication resources. Communication service providers face the constant challenge of serving their many customers, many of whose activities consume significant resources, with the infrastructure and communication spectrum available to them. Adding infrastructure to meet increasing demand is costly. In addition, if the spectrum required by the demands of users is greater than the spectrum available to meet those demands, increasing infrastructure will not meet those demands.
To avoid the costs of adding infrastructure, and to help insure that the available spectrum will meet the demands placed upon it, service providers seek to use their available resources as efficiently as possible.
Management of communication resources involves adapting signals to the capacity of a communication channel, with a communication channel typically being defined by governments or consortiums of communication operators. A bandwidth for a communication channel is defined, and insuring that signals remain within the capacity of the channel is accomplished by filtering at a transmitting device and a receiving device. FIG. 1 illustrates a diagram 100 showing an overview of signal generating and processing elements that may accomplish transmission of a signal using a defined channel capacity. In broad terms, the elements are an interpolation element 102, a transmitting filter 104, a communication channel 106, a receiving filter 108, and a decimation element 110. The transmitting filter 104 restricts the transmitted signal to the limited bandwidth provided by the communication channel 106 and the receiving filter 108 separates channel transmission from interference coming from outside of the channel 106. To compensate for signal losses that can be expected as a signal passes through the transmitting filter—communication channel—receiving filter chain, the interpolation element 102 adds redundant information to the transmitted and the decimating element 110 extracts original data from the received signal.
In conventional communication systems, filters at transmitting and receiving devices are matched to achieve the maximum possible efficiency, with a transmitting filter passing the same frequencies as a receiving filter. However, various motivations may interfere with the ability to achieve desired efficiency by matching filtering. One area of interest is the management of multiple carriers. One technique for transmitting a higher density of information within a geographic area is to use multiple carriers within the same frequency band. In order to prevent interference between carriers, the carriers are separated in frequency. A rolloff region around each carrier is employed to provide for transition between carriers. Each rolloff region includes usable signal information. To insure that each transmission stays within its own channel, regulatory bodies such as the Third Generation Partnership Project (3GPP) and the United States Federal Communications Commission (FCC) and similar governmental and industry organizations define spectral mask requirements.
FIG. 2 illustrates an exemplary set of adjacent carriers, using a mask definition providing for no overlap or minimal overlap between carriers. The carriers are illustrated in a graph 200, showing power as a function of frequency for a first carrier 202 and a second carrier 204. The first carrier is subject to filtering that imposes rolloff regions 206 and 208 that begin a prescribed distance from a center frequency 210 and behave in specified ways as the distance from the center frequency 210 increases. The present discussion is directed to an arrangement of a first carrier and a second carrier, such as the first carrier 202 and the second carrier 204, and modification of rolloff regions of the first carrier, such as the rolloff regions 206 and 208, so that discussion of rolloff behavior of the second carrier 204 and other exemplary second carriers is omitted here for simplicity.
One way to test the efficiency of a communication system is through the transmission of an impulse signal. If transmission is efficient, transmission of an impulse signal will result in a single peak after decimation, as illustrated by FIG. 3, which shows a graph 300 showing a single peak 302. Transmission of a set of carriers such as those illustrated at FIG. 2 is easy to manage efficiently, because the first carrier 202 is not reduced to accommodate the second carrier 204, so that a network operator using filtering that produces a set of carriers such as those of FIG. 2 can expect that user devices will use similar filtering. However, achieving an efficient signal becomes more difficult if an operator desires to reduce the bandwidth occupied by the carriers.
Government regulatory bodies typically define the maximum frequency range allocated to an operator, but the defined maximum frequency range often allows an operator to use a smaller frequency separation between carriers than illustrated above. Therefore, in order to increase efficiency, an operator may use a second carrier at a frequency that is within the rolloff region of a first carrier. FIG. 4 illustrates a graph 400, showing the first carrier 202, with the rolloff regions 206 and 208 and the center frequency 210, but with a second carrier 404 at a frequency overlapping the rolloff region 208. In the example illustrated here, the overlap between the rolloff region 208 and the carrier 404 produces interference with the carrier 202 and the carrier 404, so that narrowing of the rolloff regions is needed to prevent such interference.